The present invention relates to power line fault detector circuits.
Three conductor power lines such as used in home wiring and in many industrial applications include "hot" and "neutral" lines, across which approximately 120 VAC is applied, and a third, "ground" line. Both the neutral and ground lines are connected to earth ground. The ground line is essentially a duplicate of the neutral line and is provided for safety purposes; it will normally carry no current.
This ground line will be connected to the chassis of the powered equipment or any other metallic parts which are exposed to possible contact by the operator. Then, in the event of an internal short within the mechanism, the short will be shielded from the operator by the grounded chassis. It is therefore important for safety reasons that this ground line not become accidentally disconnected from earth ground. In the event that it is disconnected, the operation of electrical devices from that power line becomes unsafe and should be discontinued until the ground line is repaired.
In the past, certain electrical apparatus known as ground or power line fault detector circuits were provided for connection to the power lines to sense whether or not the ground was continuous. In its simplest form, this ground fault detector took the form of simply a resistor and a neon light interconected in series across the hot and ground lines. If the hot and ground lines were properly connected, then the neon light would glow. If the ground line was discontinuous, however, then the neon light would not glow. This approach was somewhat disadvantageous, however, in that the neon light would also glow if the hot and ground lines were interchanged; a very dangerous condition, indeed.
Another approach was to intereconnect series connected combinations of resistors and neon lights between each different pair of conductors (i.e., hot-neutral, neutral-ground, ground-hot). By examining the combination of lights that were illuminated, one could determine whether or not the power line was properly connected. This approach was not, however, convenient to utilize in industrial applications, since it required that the operator both examine the lights and know the proper combination of lights in order to determine whether or not the circuit was properly connected. These devices are designed for intermittent use as stand-alone testers, and are adapted for connection to the power line in place of the appliance connection when the power line is to be tested for integrity.